The present invention relates to methods and apparatus for modulating and/or deflecting an incident beam of coherent radiation in the optical or infrared range at very high frequency rates in a semiconductor device.
Reference may be made to the following U.S. Pat. Nos. 3,331,036; 2,836,652; 3,736,045; 3,126,485; 3,102,201; 3,624,406; and 3,586,872.
The adaptation of thin film light guides for computer read-out techniques in data processing and communication systems is considered to be highly desirable by those in the data processing and communication industries. It is basic, therefore, that practical and economical means must be provided for deflecting incident light beams of coherent radiation in the visible and infrared ranges as well as for modulating these so-called "light" beams without removing the beam from the thin film light guide.
In their paper entitled "Geometrical Optics in Thin Film Light Guides", published September, 1971, in Applied Optics, Vol. 10, No. 9 at p. 2077, R. Ulrich and R. J. Martin demonstrated that an incident light beam being transmitted or propagated through a thin film light guide can be reflected or deflected in accordance with Snell's Law.
In their experiments, a thin film light guide comprising a thin, transparent optical material having a high refractive index was sandwiched between a supportive substrate and another layer of optical material, each having a lower refractive index than the thin film. The cross section of the film used was non-uniform in thickness, having a uniform thickness throughout a first portion or region of the film and a greater but uniform thickness in a second region thereof. The two regions of the film were joined at a common boundary where the film was tapered smoothly from the first region to the second region.
Ulrich and Martin showed that a guided light beam having an initial direction obliquely incident on the boundary between the two regions of different thickness in the thin film light guide traversed the tapered region along a curved path and proceeded again in a straight path only upon existing the tapered boundary and entering the second region, the angle of exit being different with respect to the boundary than the angle of incidence. It was found that this phenomenon obeys Snell's Law: EQU N'sin .theta.' = N"sin .theta."
Where N' is the effective refractive index in the first region, N" is the refractive index in the second region, .theta.' is the angle of incidence and .theta." is the angle of exit, i.e., angle of refraction.
Accordingly, it has been found possible to form "lenses" and "prisms" in thin film light guides by varying the thickness of the film deposited on a substrate to modify the phase velocity of light traveling through the device so that it is deflected.
If the angle of the beam's incidence exceeds the critical angle, .theta..sub.c, and the beam is directed to the boundary through the second region (of greater thickness), however, the beam is totally reflected from the tapered boudary back into the second region in accordance with the geometrical optical law: EQU .theta..sub.r " .pi. - .theta..sub.i
where .theta..sub.i is the angle of incidence of the incident radiation and .theta..sub.r is the angle of reflection of the reflected radiation.
The application of thin film light guides such as those described above to read-out techniques is limited, however, since a fixed pre-determined ratio exists between the refractive index of the first region and the index of the second region corresponding to the ratio of their respective thicknesses. These characteristics are determined at manufacture and cannot be changed thereafter.
Another method of beam deflection and modulation is suggested by J. H. McFee, R. E. Nahory, M. A. Pollack and R. A. Logan in their paper entitled "Beam Deflection and Amplitude Modulation of 10.6-.mu.m Guided Waves by Free-carrier Injection in GaAs-AlGaAs Heterostructures" published Nov. 15, 1973 in Appl. Phys. Lett., Vol 23, No. 10 at p. 571. There, it was suggested that free-carrier injection could be utilized for effective amplitude and deflection modulation of 10.6-.mu.m guided waves in GaAs-AlGaAs heterostructures by addition of a suitable AlGaAs contacting layer to make a double heterostructure for electrically injecting the carriers into the GaAs-AlGaAs heterostructure.